Stirrers for minimizing erosion of refractory metal vessels in a glass making system

ABSTRACT

Stirring apparatuses for stirring molten glass are disclosed. The method includes stirring a molten glass with a stirrer comprising a layer containing at least about 50% iridium. An apparatus comprising an iridium-containing layer is also presented. In one embodiment, an apparatus for stirring molten glass includes a cylinder comprising a bore. A stirrer may be disposed in the bore. The stirrer may include a platinum or platinum alloy shaft coaxial with the cylinder. A plurality of impellers may project radially from the shaft into close proximity of a wall of the cylinder. Each impeller may include an arcuate distal end portion farthest from the shaft. The distal end portion of each impeller consists of iridium or an iridium alloy and the remainder of the impeller consists of platinum or a platinum alloy. The stirring apparatuses reduce metal loss from the refractory metal of the stirring apparatus.

This application is a divisional application and claims the benefit ofpriority to U.S. patent application Ser. No. 11/643,059, now U.S. Pat.No. 8,256,951 filed on Dec. 21, 2006, the content of which is reliedupon and incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to an apparatus for stirring glass,and in particular to an apparatus for stirring glass in transit from amain supply body to a working body or to a forming apparatus.

2. Technical Background

Formed glass is often considered to be a relatively inert material.Indeed, for this reason glass vessels often serve as containers in avast array of different industries. However, during the glassmanufacturing process molten glass is conveyed at very high temperature(in excess of 1600° C. in some cases). At such high temperatures moltenglass itself can be quite corrosive, thus requiring acorrosion-resistant system of piping and containment. This corrosion canlead to failure of the vessel material. Consequently, most containmentand transfer systems for molten glass rely upon vessels constructed fromrefractory metals. One such vessel is the stirring chamber.

In a typical glass manufacturing process, glass precursors, or batchmaterials, are combined and melted in a furnace to form molten glass(the “melt”). The glass stream flowing from the batch-melting tank orother vessel may vary in refractive index both longitudinally andtransversely at any given time. Longitudinal variations generally resultfrom changes in the batch and in the melting conditions; transversevariations generally result from volatilization of molten glassconstituents and from corrosion or erosion of the melting-containerrefractories and present themselves in the form of cords or striae.

The presence of such variations is of no particular significance in theproduction of many types of glassware. When glass designed forophthalmic or other optical purposes is being melted, however, thepresence of such variations assumes primary importance since the qualityand, hence, the saleability of the resulting ware are controlledthereby; and the reduction or substantial elimination of such variationsbecomes not only desirable but essential if satisfactory ware, i.e.,ware in which the degree of homogeneity or variation of refractive indexwithin an individual piece is maintained within a desired degree oftolerance, is to be produced.

By careful control of the batch composition together with maintainingsubstantially constant melting conditions, longitudinal variation of therefractive index can be held within a relatively narrow tolerance.

Through use of a homogenizing or stirring process cords or striaepresent in the glass can be substantially eliminated.

During the stirring process, the stirring apparatus stirs the moltenglass and breaks the cord into increasingly finer divisions until whatcord has not been homogenized into the melt is of inconsequential size.

As with the other molten glass conveying portions of the glass makingprocess, the stirring apparatus, and in particular the rotating stirrer,is typically constructed from a refractory metal capable of withstandingthe high temperature, corrosive environment of the molten glass. Therefractory metal generally chosen for this application is typicallyplatinum, or a platinum rhodium alloy.

In spite of its advantages, however, platinum, or alloys thereof, is noterosion proof, and the high stress developed within the stirringapparatus during the stirring of the viscous molten glass may lead toerosion of the metal surface and subsequent contamination of the moltenglass with refractory metal particulate.

What is needed is a method of reducing erosion of the refractory metalsurfaces of the stirring apparatus, and in particular, the rotatingstirrer.

SUMMARY

It is an object of the present invention to provide an apparatus forreducing the occurrence of refractory metal particulate in molten glass.

In one embodiment according to the present invention, a method ofstirring molten glass is disclosed comprising flowing molten glassthrough a stirring vessel, the stirring vessel comprising a stirrerdisposed therein rotating the stirrer to stir the molten glass andwherein the stirrer comprises an outer layer comprising at least about50% by weight iridium.

In another embodiment, an apparatus for stirring molten glass ispresented comprising a cylinder comprising a bore, a shaft coaxial withthe cylinder disposed within the bore, an impeller projecting radiallyfrom the shaft into close proximity of a wall of the cylinder andwherein the impeller comprises an outer layer in contact with the moltenglass comprising at least about 50% by weight iridium.

In still another embodiment, a stirrer for stirring molten glass isdisclosed comprising a shaft, an impeller extending radially from theshaft; and wherein the stirrer comprises an outer layer comprising atleast about 50% by weight iridium.

In yet another embodiment, an apparatus for stirring molten glass isdisclosed comprising a cylinder comprising a bore, a shaft coaxial withthe cylinder disposed in the bore, an impeller projecting radially fromthe shaft into close proximity of a wall of the cylinder and whereinsurfaces of the apparatus comprise a plurality of grooves.

In another embodiment, an apparatus for stirring molten glass isdisclosed comprising a cylinder comprising a bore, a shaft coaxial withthe cylinder disposed in the bore, an impeller projecting radially fromthe shaft into close proximity of a wall of the cylinder and wherein theimpeller blade is adapted to reduce a flow of molten glass over asurface thereof.

Modifications to the cylinder may be undertaken, without, or inconjunction with, similar modifications to the stirrer (e.g. shaftand/or impeller blades).

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate an exemplary embodiment of theinvention and, together with the description, serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a stirring chamber showing a stirrerdisposed therein and FIG. 1A is a cross sectional view of a portion ofthe stirring chamber of FIG. 1 showing grooves in the inside surface ofthe cylinder.

FIG. 2 is a side view of the stirrer of FIG. 1.

FIG. 3 is a partial top down view of a group of impeller blades disposedon the stirrer of FIG. 2.

FIG. 4 is a partial perspective view of a group of impellers of FIG. 3.

FIG. 5 is a partial perspective view, in cross section, of a portion ofan impeller showing an iridium or iridium alloy layer on an outsidesurface thereof.

FIG. 6A is a partial perspective view, in cross section, of a portion ofan impeller showing an iridium or iridium segment inserted into theimpeller and joined to non-iridium-containing portions thereof.

FIG. 6B is another view of a portion of the impeller of FIG. 6 a showingan inserted iridium or iridium alloy segment.

FIG. 7 is a partial perspective view of a portion of an impeller shownin FIG. 4, modified with grooves on a surface thereof.

FIG. 8A is a partial cross sectional view of an impeller showing anon-periodic pattern of grooves on a surface of the impeller.

FIG. 8B is a partial cross sectional view of an impeller showing aperiodic pattern of grooves on a surface of the impeller.

FIG. 9 a partial perspective view of a portion of an impeller shown inFIG. 4, modified with holes on a surface thereof.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, example embodiments disclosing specific details are setforth to provide a thorough understanding of the present invention.However, it will be apparent to one having ordinary skill in the art,having had the benefit of the present disclosure, that the presentinvention may be practiced in other embodiments that depart from thespecific details disclosed herein. Moreover, descriptions of well-knowndevices, methods and materials may be omitted so as not to obscure thedescription of the present invention. Finally, wherever applicable, likereference numerals refer to like elements.

Illustrated in FIG. 1 is a stirring apparatus 10 according to anexemplary embodiment of the present invention. Stirring apparatus 10comprises an inlet tube 12 extending between a supply of molten glass(not shown) and a cylinder 14. Inlet tube 12 and cylinder 14 may besurrounded by electrical heating windings 16 and 18, and insulated toprevent excessive heat loss. The stirred and homogenized molten glassissues from cylinder 14 through outlet tube 20, which, although notshown, may also be surrounded by electrical heating elements.

The diameter of outlet tube 20 may be that size deemed most suitable forflowing the molten glass, depending upon the viscosity of the glass, theparticular style of stirrer 22 arranged within cylinder 14, therotational speed of the stirrer, and the diameter of the stirrerrelative to the inside wall diameter of the cylinder bore.

Stirrer 22, rotatably disposed within the bore of cylinder 14, may takeany number of forms. It has been found that the most efficient stirrermay not be the best stirrer for all operating conditions. Completehomogenization of the molten glass can be obtained only if the entirebody of glass is forced to pass through a zone of turbulence where thedifferent portions of the glass are progressively sheared with respectto one another, and cords and inhomogeneities are attenuated anddispersed through the parent glass. Desired results can best be obtainedby passing the glass through a stirring chamber, preferably cylindricalin shape, having a longitudinally extending stirrer disposed thereinwhose maximum diameter is only slightly less than that of the cylinderbore. Preferably, stirrer 22 is disposed concentrically within cylinder14. That is, the axis of rotation of stirrer 22 is coincident with thecentral longitudinal axis of cylinder 14. Even under thesecircumstances, certain precautions must be taken to prevent cords of theinhomogeneous glass from creeping along the inner wall of the cylinderor along the shaft of the stirrer without becoming mixed with the mainbody of the glass. These cords are termed wall cords and shaft cords,respectively. As used herein, cord refers to inhomogeneous regions ofthe glass wherein the refractive index of the glass locally varies. Onecause of cord is incomplete mixing of the glass.

Different devices may be employed to eliminate shaft cords and wallcords. For example, shaft cords may be eliminated by designing thestirrer so that it embodies a mechanical obstacle to such cords whichwill force glass tending to flow along the shaft out into a zone ofturbulence where it is subject to the shearing and mixing action of thestirrer blades.

Wall cords, on the other hand, may be reduced by the closeness of thecoupling between the outer edge of the stirrer and the inner wall of thecylinder (stirring chamber). Wall cords may be completely eliminated byestablishing a dynamic dam of glass which is forced outwardly by thestirrer impeller against the cylinder wall at a rate sufficiently greatthat the glass divides and at least some of the glass forms acounterflow along the chamber wall in opposition to the normal directionof flow of glass through the apparatus.

The effectiveness of apparatus 10 is also influenced by the speed ofrotation of stirrer 22. Thus, the glass capacity of the equipment (asmeasured by the rate of through flow of the molten glass) is determinednot only by its dimensions, but also by the speed of rotation of thestirrer deposited therein. If the rate of flow of the molten glassthrough the apparatus is kept low, such as when the apparatus outlet isconstricted, or the glass viscosity is high, a speed of rotation of arelatively few revolutions per minute will suffice to appropriately mixthe glass and dynamically disrupt the continuity of a downward flow ofglass along the cylinder wall. On the other hand, as the outlet isenlarged and the flow rate increased, the speed of the stirring actionmust also be increased to maintain the dynamic condition for productionof the desired glass quality.

As illustrated in FIGS. 1 and 2, exemplary stirrer 22 comprises a shaft24 driven by pulley 26, and further comprising groups 1, 2 and 3,respectively, of arcuate shaped impellers arranged along the length ofthe shaft. Turning to FIG. 3, each group of impellers 1, 2, 3 comprisesa set of three impeller blades, such as impeller blades 28-30, arrangedadjacent a similar set of three impeller blades 31-33 curved in theopposite direction. It should be noted that the number of groups ofimpellers, and the number of impeller blades per group may varyaccording to need. For example, stirring apparatus 10 may have more orless than three groups of impellers.

Each of the impeller blades 28-33 has its major surface area parallel tothe axis of rotation of shaft 24, and each major surface area is arcuateand in some embodiments is comprised of at least a portion of acylinder. Each impeller blade 28-33 also has one end (the proximate end)suitably anchored to shaft 24 and each impeller blade is anchored to anoppositely curved impeller blade to provide rigidity to the impeller. Toavoid confusion, as described hereinafter each assembly comprising oneimpeller blade anchored to another impeller blade constitutes animpeller. Thus each impeller comprises two impeller blades joined toform an arcuate shape that is substantially circular in aspect. Inaccordance with the present exemplary embodiment depicted in FIG. 2-3,each impeller set or group 1, 2, 3 includes three impellers 13, 15 and17. As will be evident to those skilled in the art, other stirrerdesigns as are known in the art may be used in conjunction with theteaching of this disclosure. Alternative stirrer designs include, butare not limited to designs employing a helical screw and stirrersemploying paddles at a given angle relative to the direction of rotationof the stirrer.

To add further rigidity to each impeller and further promote theirstirring action, rigidly secured web members in the form of disksegments 34-39 may be provided. Disk segments 34-39 are arranged normalto the axis of shaft 24, follow the inner curvature of the impellerblades and overlap one another when viewed in a vertical manner, thusobstructing direct downward flow of glass within the cylinder volumeoccupied by the stirrer. As will be appreciated, during rotation ofstirrer 22 the impeller blades of some impeller sets throw the glassoutward while the impeller blades of the remaining sets pull the glassinward, thereby thoroughly mixing the glass. In short, the movementimparted to the glass by the stirrer is such that not only is the glassthoroughly mixed but, in addition, the flow of glass down the cylinderside wall is effectively prevented by a dynamic dam to such flow createdby pressures built up along the wall area.

While this type of stirrer is most efficient if each group of impellerson the stirrer shaft is made up of two or more impellers each having twoor more oppositely curved blades, one or more sets of impellers with theimpeller blades all curved in the same direction will stir glasssatisfactorily when a relatively limited output of glass meetsproduction requirements. Indeed, as stated, the benefits of the presentinvention may be derived from a variety of different stirrer designs,and the application thereof should not be construed to be limited to anyone particular design.

Experimentation and post-mortem examination of failed stirrers has shownthat the outside surface of the stirrer impeller/impeller blades issubjected to high stress. This stress results, in part, from theviscosity of the molten glass flowing through the stirring apparatus,and the close tolerance between the stirrer blades and the stir chamberwall. It has been found that the outer-most portions of the impellersmay be subject to some of the highest stresses during the stirringprocess, although forward facing surfaces of the stirrer (relative tothe direction of rotation of the stirrer) and the wall portions adjacentto the impellers may also be subject to high stress. That is, theportion of each impeller which is farthest from shaft 12 and closest tothe inside wall of cylinder 14, and those portions of cylinder 14directly opposite the sweep of stirrer 22, generally experience higherstress than other portions of apparatus 10.

Molten glass to which portions of apparatus 10 are exposed may exceed1000° C. It can therefore be appreciated that materials comprisingapparatus 10 must be capable of withstanding such high temperatures.Moreover, in addition to being resistant to the temperatures of moltenglass, apparatus 10 must exhibit both corrosion and erosion resistanceunder the aforementioned stresses. For this reason, various componentsof apparatus 10, particularly those in contact with the molten glass,comprise refractory metals known to provide a degree of protection fromharsh environments.

In spite of the use of suitable refractory metal materials in theconstruction of apparatus 10, it has been found that the high stress towhich apparatus 10 may be subjected can be responsible for the releaseof refractory metal particles into the molten glass. It is believed thisparticle release occurs due to erosion of the refractory metal. Theserefractory metal particles may eventually make their way into thefinished glass product as defects. Depending upon the application forthe glass, such defects may make the glass unusable. For example,optical applications, such as LCD display devices, are highly intolerantof defects.

In some embodiments according to the present invention, stirrer 22 maycomprise a core portion which has been formed from a suitable refractorymaterial, and thereafter coated or clad with a refractory metal. Forexample, in some applications, the core of stirrer 22 may be formed frommolybdenum, and thereafter coated with a refractory metal comprisingplatinum. The molybdenum core provides shape and mechanical strength tothe stirrer, whereas the molybdenum outer layer provides wear andcorrosion resistance. In other embodiments, the core of the stirrer maybe formed entirely of platinum, or a platinum alloy, such as aplatinum-rhodium alloy. Platinum in particular is a desirable refractorymetal for glass stirring applications because of its high melting point,corrosion resistance and workability. Nevertheless, platinum, or evenplatinum rhodium alloys, are not immune to erosion during hightemperature stirring processes. By incorporating a more wear resistantmaterial such as iridium, particulate originating from the stirringoperation can be significantly reduced.

Several approaches to the application of iridium in stirring apparatus10 may be employed. In one such approach, depicted in FIG. 5, iridium,or an iridium alloy (e.g. an iridium-rhodium alloy) may be formed ontocore portion 19 as layer 21. In this respect, the core is defined asthat base material over which a layer of different material is placed.FIG. 5 illustrates a partial perspective view of a portion of animpeller (e.g. impeller 13), in cross section, and shows core portion 19and an outer layer 21. For example, a conventional stirrer, such as aplatinum or platinum rhodium alloy stirrer, may serve as a core and belayered with one or more iridium or iridium alloy layers 21. Such layersmay be applied by conventional methods, such as flame or plasmaspraying, or by sputtering for example. Moreover, because the high wearregion of stirrer 22 is proximate the ends of the impellers, only aportion of the stirrer need comprise an iridium-containing layer, suchas the distal end portions 27 of the impellers (that portion of theimpellers closest to the stir chamber walls, or farthest from shaft 24).Layer 21 ideally comprises at least about 50% by weight iridium, but maycomprise up to and including essentially 100% iridium, with theunderstanding that the iridium may have some finite but small amount ofimpurities that do not detract from the purpose and function of theiridium layer.

Similarly, although not shown, the inside surfaces of cylinder 14 mayinclude a layer in contact with the molten glass comprising at leastabout 50% by weight iridium.

To provide a thicker layer than is ordinarily achievable, such as withsputtering, the platinum or platinum alloy core 19 of stirring apparatus10 may be covered with iridium or an iridium alloy layer 21 by cladding.Application of the cladding may be performed, for example, by hotpressing as is known in the art. The portions of stirring apparatus 10which may be clad include the inside surface of cylinder 14, and theimpellers of stirrer 22, and in particular the impeller blades. However,it should be understood that substantially all of stirrer 22 may includea layer 21. The cladding layer is preferably thick enough that thestirrer does not experience a substantial reduction in service life overconventional stirrer designs. The iridium or iridium alloy layer may beat least about 100 μm thick, but preferably at least about 0.5 mm thick,and may be as thick as about 2 mm.

Although FIG. 5 illustrates layer 21 as being on one side of core 19,layer 21 may be formed on one or both sides, and may, in someembodiments, fully encase core 19. Core 19 may be any suitable materialwhich provides form and strength to stirrer 22. Core 19 may, forexample, comprise molybdenum or a molybdenum alloy, a ceramic, platinumor a platinum alloy.

In still another embodiment, shown in FIG. 6 a, portions of cylinder 14and stirrer 22, and in particular portions of the impellers, may befabricated in whole from iridium or an iridium alloy, and inserted intothe apparatus, such as by welding. For example, the impeller distal ends(those portions of the impellers closest to the cylinder wall) may befabricated from iridium, or an iridium-rhodium alloy, with the remainderof the impeller and/or the stirrer fabricated from a non-iridiumcontaining material, such as platinum and/or a platinum rhodium alloy.Shown in FIG. 6A is a cross sectional slice from an impeller, e.g.impeller 13. Segment 23, comprising iridium is inserted into a portionof the impeller and joined to the platinum or platinum rhodium portions25 of the impeller, such as by welding. Cylinder 14 may also becomprised in whole or in part from iridium or an iridium alloy. Forexample, those portions of cylinder 14 swept by the impellers may beselectively fabricated from iridium or an iridium alloy, and joined toan adjacent section of the cylinder which may or may not compriseiridium. Alternatively, the entire inside surface of cylinder 14 mayinclude a layer of iridium or an iridium alloy. FIG. 6B shows a largerimpeller portion (e.g. impeller 13), rather than a cross sectional slicethereof, and depicts a central segment 23 of the impeller joined tosections 25. In accordance with the present embodiment, central segment23 is fabricated from a refractory metal comprising iridium, and theabutting segments 25 are fabricated from another refractory metal, suchas platinum or an alloy thereof. Central segment 23 may comprise atleast about 50% iridium, but in some embodiments segment 23 may beessentially 100% iridium. Suitable iridium alloys include iridiumplatinum and iridium rhodium.

To further reduce erosion of the stirrer, other modification of stirringapparatus 10 may be made, either alone or in conjunction with the use ofan iridium or an iridium alloy layer. For example, the stirrer may beadapted to produce a static, or quasi-static layer of molten glass onsurfaces of the stirrer. In particular, the distal end portions 27 ofthe impellers may comprise grooves in the outside surface thereof.Preferably, grooves 40 are aligned parallel to the long axis of thestirrer shaft and perpendicular to the flow of glass over the surfacecomprising the grooves, but may vary depending upon the stirrer design.Grooves 40 trap the viscous molten glass, creating a layer of static, orquasi static glass over the end portions of the stirrer impellers.Whereas a smooth surface as used in conventional stirrers leads to flowrelated erosion of the surface, a static layer of molten glass accordingto the present embodiment has the effect of forming a protective layerof glass over the surface of the impeller, thereby reducing impellererosion by reducing the flow of glass over the surface of the impeller.

Grooves 40 may be non-periodic in their arrangement as shown in FIG. 8A,or they may be periodic as illustrated in FIG. 8B. Grooves 40 may berectangular in shape, with sharp corners defining the groove, or thegrooves may be defined by rounded corners and/or arcuate or angledwalls. Grooves 40 may vary in size such that the width of the groovesvary. For example, the grooves may have any of the shapes shown in FIG.8A. Indeed, an impeller may comprise all or a portion of the grooves ofFIG. 8A, e.g. non-periodic, varying width and varying groove wallshapes.

Similar to the stirrer, the inside surfaces of cylinder 14 may comprisefeatures which produce a static or quasi-static layer of glass on thecylinder inside surfaces. The features may include dimples and/orgrooves 11, as depicted in FIG. 1A. For example, grooves 11 may be castor machined into the inside surface of cylinder 14. The grooves 11 maybe consistent with the grooves described supra for impeller surfaces.That is, the grooves 11 may be aligned parallel with shaft 24,perpendicular with the direction of rotation or sweep of stirrer 22,such that the grooves 11 are substantially perpendicular with a flow ofmolten glass over the surface of the cylinder, and have similar shapesand spacing. Other orientations for the cylinder grooves 11 are alsopossible, such as angled or skewed such that the grooves 11 areanalogous to rifling in a gun barrel, as is appropriate for the style ofstirrer used. If angled, the grooves may have a right hand twist or aleft hand twist. The grooves 11 may be periodic, or non-periodic. In theinstance of dimples (not shown), the dimples may be indented (away fromthe impellers), or protruding (toward the interior of cylinder 14.Dimples may be imparted to portions of stirrer 22 as well, such as theimpellers.

In another embodiment, the impellers are machined or otherwise modifiedsuch that they define holes extending through the thickness of theimpellers, such as depicted in FIG. 9. Through holes 42 are preferablyformed at and proximate to the distal end portions 27 of the impellers,but may cover a substantial portion of the total surface of the arcuateimpeller blades. Through holes 42 produce a similar effect as grooves40, establishing a static or quasi static layer of glass at the end ofthe impellers, thereby reducing erosion of the impellers. Moreover, theelimination of material which would otherwise be available to createparticulate also contributes to a reduction in released particulate. Thenumber of holes, the diameters of the holes and the arrangement thereof,should not be such that the stirrer is unacceptably weakened. Preferablythe holes have a diameter less than about 1 cm each. Moreover, thegrooves or holes need not extend over the entire surface of theimpellers, but may be limited to a region which extends less than about5 cm in either direction from the point farthest from the stirrer shaft.

It should be emphasized that the above-described embodiments of thepresent invention, particularly any “preferred” embodiments, are merelypossible examples of implementations, merely set forth for a clearunderstanding of the principles of the invention. Many variations andmodifications may be made to the above-described embodiments of theinvention without departing substantially from the spirit and principlesof the invention. For example, the surfaces of the impeller blades maybe dimpled, or otherwise modified or modulated, so as to create a staticor quasi-staic layer of glass on a surface of the impeller blades,thereby reducing or eliminating wear of the surface in accordance withprinciples of the present invention. Other stirrer designs (not shown)may include a spiral or screw configuration. All such modifications andvariations are intended to be included herein within the scope of thisdisclosure and the present invention and protected by the followingclaims.

What is claimed is:
 1. A stifling apparatus for stirring molten glasscomprising a cylindrical vessel; a stirrer rotatably disposed within thecylindrical vessel, the stirrer comprising: a shaft; an impeller bladecoupled to the shaft and extending outward therefrom, the impeller bladecomprising a cylindrical member parallel to an axis of rotation of theshaft; and wherein a distal end portion of the cylindrical memberadjacent an inner surface of the cylindrical vessel defines a pluralityof holes extending through the distal end portion, the plurality ofholes being limited to a region of the distal end portion less than 5 cmin either circumferential direction from a point in the distal endportion farthest from the stirrer shaft.
 2. The stifling apparatusaccording to claim 1, wherein a diameter of the plurality of holes isless than 1 cm each.
 3. The stifling apparatus according to claim 1,further comprising a web member coupled to and perpendicular to theshaft, the web member being further coupled to the cylindrical member.4. The stifling apparatus according to claim 1, wherein the distal endportion comprises iridium.
 5. The stifling apparatus according to claim4, wherein the distal end portion consists of iridium.
 6. The stirringapparatus according to claim 1, wherein a wall of the cylindrical vesselcomprises iridium.